The present invention pertains to a process for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it as well as to a corresponding control device.
The parameters of breathing gas supply by the respirator, e.g., the respiration pressure, the breathing gas flow or the respiration rate, are usually set in advance and are then checked for their validity at greater time intervals. If the patient""s respiration need changes, this is then taken into account during the treatment when the parameters of the breathing gas supply are correspondingly adapted after a recent check of the respiration need. A breathing gas supply is therefore obtained that may not be oriented uninterruptedly on the patient""s current needs.
DE 198 08 543 C2 discloses a process by which the mechanical properties compliance, resistance and, finally, the muscle pressure of the respiratory system of a patient can be determined during both spontaneous breathing and mechanical respiration, and the patient""s breathing is disturbed only minimally. The compliance is the quotient of the displaced breathing gas volume and the change in the airway pressure, and the resistance is the quotient of the change of the airway pressure and the change in the breathing gas flow. The muscle pressure is defined as the percentage of the airway pressure caused by the patient""s own effort. This process makes it possible to perform the continuous determination of the muscle pressure of the patient and thus to monitor the intensity of his spontaneous breathing. An occlusion, during which the respiration or spontaneous breathing of the patient is interrupted by closing corresponding valves on a respiration tube leading to the patient, is brought about for this purpose for a short time at different points in time during the individual breathing strokes. The mechanical properties of the lungs can be determined from the measured values for the airway pressure and the breathing gas flow. The muscle pressure is in turn determined from these variables.
The object of the present invention is to provide a process as well as a device for controlling at least one parameter of the breathing gas supply for a patient, which takes into account the current state of his respiratory system.
According to the present invention, the measured values for the airway pressure, PAW, and the breathing gas flow, d/dt V, are used continuously for the determination of the mechanical properties of the patient""s respiratory system, i.e., the resistance RL and the compliance C of his lungs, and, finally for the determination of the muscle pressure PMUS. 
According to the invention, a process is provided for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it. With a brief disturbance of the breathing gas supply taking place during a first phase of expiration, the airway pressure PAW and the breathing gas flow d/dt V are measured before and during the disturbance, and the resistance RL of the patient""s lungs is determined as the quotient of the airway pressure PAW and the change in the breathing gas flow d/dt V. The airway pressure PAW and the breathing gas flow d/dt V are measured during a second, undisturbed phase of expiration, and the compliance C of the patient""s lungs is determined from this as well as from the resistance RL. The breathing gas flow d/dt V is measured each time during the subsequent breaths, and an airway pressure PAWC is calculated from this together with the resistance RL and the compliance C. A muscle pressure PMUS is determined as the difference between the calculated airway pressure PAWC and the actually measured airway pressure PAW. The at least one parameter of the breathing gas supply is changed such that the breathing gas supply is increased during the further phases of inspiration when the muscle pressure PMUS is above a set point P0, and the at least one parameter of the breathing gas supply is changed such that the breathing gas supply is reduced during the subsequent phases of inspiration when the muscle pressure PMUS is below the set point P0 until the muscle pressure PMUS reaches the set point P0.
According to another aspect of the invention a device is provided for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it. The device includes means designed to bring about a brief disturbance in the breathing gas supply during a phase of expiration. A first sensor is provided for measuring the airway pressure PAW and a second sensor is provided for measuring the breathing gas flow d/dt V. An evaluating and control unit is provided for determining the resistance RL and the compliance C of the patient""s lungs from the airway pressure PAW measured by the first sensor and the breathing gas flow d/dt V measured by the second sensor, for calculating an airway pressure PAWC from the resistance RL, the compliance C and the measured breathing gas flow d/dt V, and for determining a muscle pressure PMUS as the difference between the calculated airway pressure PAWC and the actually measured airway pressure PAW. The at least one parameter of the breathing gas supply is changed by the evaluating and control unit such that the breathing gas supply is increased during the subsequent phases of inspiration when the muscle pressure PMUS is above a set point P0, and the at least one parameter of the breathing gas supply is changed by the evaluating and control unit such that the breathing gas supply is reduced during the subsequent phases of inspiration when the muscle pressure PMUS is below the set point P0 until the muscle pressure PMUS reaches the set point P0.
According to another aspect of the invention a process is provided for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it. The airway pressure PAW and the breathing gas flow d/dt V are measured in a plurality of phases of expiration, and the resistance RL and the compliance C of the patient""s lungs are determined from this. The breathing gas flow d/dt V is measured during subsequent breaths and an airway pressure PAWC is calculated from this together with the resistance RL and the compliance C. A muscle pressure PMUS is determined as the difference between the calculated airway pressure PAWC and the actually measured airway pressure PAW. The at least one parameter of the breathing gas supply is changed such that the breathing gas supply is increased during the subsequent phases of inspiration when the muscle pressure is above a set point P0, and the at least one parameter of the breathing gas supply is changed such that the breathing gas supply is reduced during the subsequent phases of inspiration when the muscle pressure PMUS is below the set point P0 until the muscle pressure PMUS reaches the set point.
In the process for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it, the breathing gas supply is briefly disturbed during a first phase of expiration. The airway pressure PAW and the breathing gas flow d/dt V are measured before and during the disturbance. The resistance RL of the patient""s lungs is determined from this. The airway pressure PAW and the breathing gas flow d/dt V are likewise measured during a second, undisturbed phase of expiration. The compliance C of the patient""s lungs is determined from these values as well as the resistance RL determined in the preceding step.
The breathing gas flow d/dt V is measuring during the subsequent breaths. Together with the values determined before for the resistance RL and the compliance C, an airway pressure PAWC is calculated from this.
The muscle pressure PMUS, i.e., the percentage of the airway pressure caused by the patient""s own effort, is subsequently determined as the difference from the calculated airway pressure PAWC and the actually measured airway pressure PAW. The control proper is now performed as a function of the muscle pressure PMUS determined. The at least one parameter of the breathing gas supply is now changed such that the breathing gas supply is increased during the subsequent phases of inspiration when the muscle pressure PMUS is above a set point P0, and the at least one parameter of the breathing gas supply is changed such that the breathing gas supply is reduced during the subsequent phases of inspiration when the muscle pressure PMUS is below the set point P0 until the muscle pressure PMUS reaches the set point P0.
If, e.g., a positive value is thus obtained for PMUS during the phases of inspiration and a PMUS value close to zero is obtained during the phases of expiration, this indicates that a higher respiration pressure or breathing gas flow is necessary for an increased breathing gas supply. If, moreover, PMUS is already positive before the beginning of a phase of inspiration, an increase in the respiration rate may be expedient.
In a preferred embodiment of the process, the at least one parameter is the respiration pressure, i.e., the airway pressure generated by the respirator. Moreover, the breathing gas flow generated by the respirator as well as the respiration rate preset by the respirator may be used as parameters. The brief disturbance in the breathing gas supply of the patient may be an occlusion of the patient""s airways, but also a change in the respiration pressure.
The device according to the present invention for controlling at least one parameter of the breathing gas supply of a respirator with a patient connected to it is designed to bring about a brief disturbance in the breathing gas supply during a phase of expiration. It has a first sensor for measuring the airway pressure PAW and a second sensor for measuring the breathing gas flow d/dt V. Moreover, an evaluating and control unit is used to determine the resistance RL and the compliance C of the patient""s lungs from the airway pressure PAW measured by the first sensor and the breathing gas flow d/dt V measured by the second sensor. The evaluating and control unit calculates an airway pressure PAWC from further measured values for the breathing gas flow d/dt V as well as from the resistance RL determined previously and the compliance C. Finally, the muscle pressure PMUS, i.e., the percentage of the airway pressure caused by the patient""s own effort, is determined. It is determined by the evaluating and control unit as the difference between the calculated airway pressure PAWC and the actually measured airway pressure PAW. The evaluating and control unit then changes the at least one parameter of the breathing gas supply in the manner already described above.
The disturbance in a phase of expiration of the patient is always brief if possible and is, e.g., in the range of a few hundred msec, preferably about 200 msec, in order not to compromise the patient""s respiration. The disturbances are performed in a sequence following a chronological pattern and occur, e.g., once or several times per minute. It is advantageous in this connection to induce the individual disturbances at different points in time relative to the individual phases of expiration, because the reliability of the measurements is increased by this scattering.
The present invention utilizes the situation that the muscle pressure PMUS is a meaningful value when information is to be obtained on the patient""s respiratory efforts. If the muscle pressure PMUS is maintained at a preset value, which is usually zero or very low in practice, it is guaranteed in this manner that the patient must make only little respiratory effort of his own and the so-called xe2x80x9cventilator fightingxe2x80x9d is avoided.
It will be described below how the resistance RL and the compliance C of the patient""s lungs are determined from the measured values for the airway pressure PAW and the breathing gas flow d/dt V and, finally, how the muscle pressure PMUS is determined.
The starting point is the following basic equation:       PMUS    =                            1          C                ·        V            +                                    R            L                    ·                      ⅆ                          /                              ⅆ                t                                                    ⁢                  xe2x80x83                ⁢        V            -      PAW      +              PEEP        t              ,
in which V designates the displaced breathing gas volume. PEEPt=PEEPIN+PEEPEX is the total positive end expiratory pressure, which is composed of the intrinsic PEEPIN, which results from the gas volume left in the patient""s lungs after complete expiration, and the external PEEPEX, which is predetermined by the respirator. (For dividing the total PEEPt into the two components PEEPIN and PEEPEX, see, e.g., U.S. Pat. No. 6,015,388.)
The resistance RL is first determined as a quotient of the difference of two measured airway pressures PAW1 and PAW2 and the difference of two breathing gas flows measured at the same points in time (d/dt V)1 and (d/dt V)2:       R    L    =                    PAW1        -        PAW2                                          (                                          ⅆ                                  /                                      ⅆ                    t                                                              ⁢                              xe2x80x83                            ⁢              V                        )                    ⁢          1                -                              (                                          ⅆ                                  /                                      ⅆ                    t                                                              ⁢                              xe2x80x83                            ⁢              V                        )                    ⁢          2                      .  
The PAW1 and (d/dt V)1 measurements are performed before and the PAW2 and (d/dt V)2 measurements during a brief disturbance in a phase of expiration. If the disturbance is an occlusion, the breathing gas flow (d/dt V)2 becomes zero, and the following is obtained for the resistance RL of the lungs:       R    L    =            PAW1      -      PAW2                      (                              ⅆ                          /                              ⅆ                t                                              ⁢                      xe2x80x83                    ⁢          V                )            ⁢      1      
The external resistance Rex and the external PEEPex are then determined from the following equation:
PAW=xe2x88x92Rexxc2x7d/dt V+PEEPEX.
The external resistance Rex is the resistance caused by the resistance in the expiration branch of the respirator, which forms the total resistance of the patient""s lungs together with the resistance RL.
The above equation is based on the basic equation for the special case of an expiration. By performing at least two measurements for the airway pressure PAW and the breathing gas flow d/dt V, a sufficient number of equations are obtained for determining the external resistance Rex and the external PEEPEX.
The more measurements that are carried out, the better are the possibilities to obtain reliable values for Rex and the external PEEPEX with approximation methods.
The following equation applies to the breathing gas flow d/dt V and the breathing gas volume VEX displaced during an expiration:             ⅆ              /                  ⅆ          t                      ⁢          xe2x80x83        ⁢    V    =                    -                  1          τ                    ⁢              V        ex              +                  (                              ⅆ                          /                              ⅆ                t                                              ⁢                      xe2x80x83                    ⁢          V                )            t0      
with the expiration time constant xcfx84 and an initial value (d/dt V)t0 at the time t0.
Several measurements of the breathing gas flow d/dt V lead to a sufficient number of equations for determining the expiration time constant xcfx84 and the initial value (d/dt V)t0 here. As in the case above, approximation solutions of a higher reliability can be determined with additional measurements. The compliance C can now be determined based on the equation   C  =            τ              Rex        +        RL              .  
The intrinsic PEEPIN is obtained from             PEEP      IN        =                            1          C                ⁢                  (                                    τ              ·                                                (                                                            ⅆ                                              /                                                  ⅆ                          t                                                                                      ⁢                                          xe2x80x83                                        ⁢                    V                                    )                                t0                                      -                          V              ee                                )                    -              PEEP        EX              ,
in which Vee is a breathing gas volume displaced during a complete expiration. The term xcfx84xc2x7(d/dt V)t0xe2x88x92Vee designates the so-called hyperinflation volume.
It is possible at this point to calculate an airway pressure PAWC using the variables determined before and with additional measured values for the breathing gas flow d/dt V. In addition, a value is obtained for the airway pressure PAW by measurement. The muscle pressure PMUS is obtained as the difference between the calculated and measured airway pressures:
PMUS=PAWCxe2x88x92PAW.
The calculation steps indicated here, which lead to a value for PMUS, have the character of an example. The basic principle of the process according to the present invention, which is also used in modified calculation steps, is the calculation of an airway pressure PAWC from the previously measured values for the airway pressure PAW and the breathing gas flow d/dt V and the mechanical parameters of the patient""s respiratory system, which were determined therefrom, the subsequent comparison with an actually measured value for the airway pressure PAW and, finally, the control on the basis of a comparison between two values, expressed as the muscle pressure PMUS. 
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.